The liquefaction of low-boiling gases at temperatures far below ambient is achieved by cryogenic refrigeration systems which utilize selected refrigerants to reach the required condensation temperatures of the liquefied gases. Appropriate refrigerants and refrigeration cycles for such systems can be selected to minimize the power requirements in energy-intensive liquefaction processes. Cryogenic processes for the liquefaction of low-boiling gases such as helium, hydrogen, methane, and nitrogen are well-known in the art.
Refrigeration for the liquefaction of these gases typically utilizes several types of refrigeration systems, often in combination, to cool feed gas to its condensation temperature. External closed-loop refrigeration systems are used which transfer heat indirectly from the gas to be liquefied. Autorefrigeration, in which the gas being liquefied is cooled directly by throttling or work expansion, is also utilized for the lowest-boiling gases such as helium, hydrogen, and nitrogen. Combinations of closed-loop refrigeration and autorefrigeration systems are used to achieve higher process efficiency.
A typical nitrogen liquefaction process compresses warm nitrogen gas to one or more pressure levels, cools the compressed gas, and work expands portion of the cooled compressed gas in one or more turbo-expanders to provide the refrigeration for liquefaction. The cooling effect produced by this work expansion step is defined as autorefrigeration. The remaining portion of the compressed gas is cooled in a heat exchanger against the cold turbo-expander discharge stream or streams, reduced in pressure, and recovered as a liquid. The use of multiple expanders which operate over different temperature levels, and often different pressure levels, improves the efficiency of the process by providing refrigeration at the most appropriate locations of the heat exchanger. The desired result is lower compressor power consumption. There are numerous examples in the art of nitrogen liquefiers of the turbo-expander type. U.S. Pat. No. 5,836,173 illustrates a single turbo-expander cycle; U.S. Pat. No. 4,778,497 and U.S. Pat. No. 5,231,835 illustrate dual turbo-expander cycles; and U.S. Pat. No. 4,894,076 and U.S. Pat. No. 5,271,231 illustrate triple turbo-expander cycles.
A typical two-expander nitrogen liquefier is shown on FIG. 16.15 of "Cryogenic Engineering" edited by B. A. Hands, Academic Press, Inc., London 1986. Refrigeration is provided by two turbo-expanders operating over two temperature levels. As illustrated in this reference, additional refrigeration at the warmest temperature level can be provided by precooling the pressurized nitrogen stream in a chiller. Such a chiller, which is typically a closed-loop freon or ammonia refrigeration unit, was commonly used in nitrogen liquefiers built through the nineteen-eighties. The use of precooling also is disclosed in U.S. Pat. No. 4,375,367. Improvements in turbo-expander efficiencies and environmental restrictions on the use of certain refrigerants have reduced the applicability of such precooling approaches. Furthermore, the temperature level achievable by precooling is modest, typically not below about -40.degree. F. (-40.degree. C.).
Refrigeration may be available from an external source in certain situations. This refrigeration can be used, for example, to provide precooling and refrigeration for the liquefaction of nitrogen. An example application is refrigeration obtained from the warming and vaporization of liquefied natural gas (LNG) for distribution and use. U.S. Pat. No. 5,139,547 discloses the use of refrigeration from vaporizing LNG in the liquefaction of nitrogen. Nitrogen liquefaction cycles based only on using refrigeration from LNG are not very efficient since the normal boiling point of methane is -260.degree. F. and the normal boiling point of nitrogen is -320.degree. F. U.S. Pat. No. 5,141,543 acknowledges this by disclosing the use of a supplemental nitrogen turbo-expander for providing refrigeration at the coldest temperatures. A striking feature of U.S. Pat. Nos. 5,139,547 and 5,141,543 is that much of the refrigeration from the vaporizing LNG is used to allow nitrogen compression at cold temperatures. This occurs because the LNG, being primarily a pure component and being vaporized at a single pressure, provides a disproportionate amount of refrigeration over a relatively narrow temperature range.
Typical natural gas liquefiers use closed-loop refrigeration cycles. The most popular of these cycles employ a mixture of components for the circulating fluid. In these processes, a multicomponent or mixed refrigerant is compressed, condensed, cooled, reduced in pressure, and vaporized. The vaporization of the mixed refrigerant provides the refrigeration needed to liquefy the pressurized natural gas. Multiple pressure levels and composition ranges often are employed for the mixed refrigerant to provide refrigeration at the most appropriate temperature levels and locations in the heat exchanger.
Numerous types of closed-loop mixed refrigerant processes are known in the art. U.S. Pat. No. 5,657,643 discloses a relatively simple single mixed refrigerant cycle which is used specifically for natural gas liquefaction or in general for cooling a fluid. Other examples of single mixed refrigerant cycles include U.S. Pat. Nos. 3,747,359 and 4,251,247. The efficiency of single mixed refrigerant cycles is limited because the required refrigeration for feed gas liquefaction must be provided over a temperature range greater than that achievable in a single mixed refrigerant cycle. In other words, it is difficult to produce a single composition of mixed refrigerant components which can efficiently provide refrigeration over a temperatures range of ambient to -260.degree. F.
The more efficient closed-loop mixed refrigerant processes use multiple refrigerant cycles to span the required temperature range more efficiently. One popular type is the propane-precooled mixed refrigerant cycle, an example of which is disclosed in U.S. Pat. No. 3,763,658. A first refrigeration loop uses propane to precool a mixed refrigerant in a second refrigeration loop, and also the natural gas feed, to approximately -40.degree. F. Other types of multiple refrigerant cycles use two different mixed refrigerant loops operating at different temperatures. These cycles, often termed "dual-mixed refrigerant" cycles, are described in U.S. Pat. Nos. 4,274,849 and 4,525,185. A third type of multiple refrigerant cycle is called a "cascade" cycle which typically uses three refrigeration loops. The warmest loop employs propane as the working fluid, the coldest loop employs methane as the working fluid, and the intermediate temperature loop uses either ethane or ethylene as the working fluid. FIG. 4.19 in "Cryogenic Process Engineering" by K. D. Timmerhaus and T. M. Flynn, Plenum Press, New York 1989 briefly describes this cycle.
Although it is theoretically possible to liquefy nitrogen by using the closed loop mixed refrigerant cycles employed to liquefy natural gas, the efficiency of such cycles would be less than desired because these mixed refrigerant systems are inefficient in supplying refrigeration at the low temperatures required to liquefy nitrogen. Improved nitrogen liquefaction processes are desirable which are more economical and efficient than the conventional processes discussed above. It is the objective of the present invention, as described below and defined by the claims which follow, to provide an improved nitrogen liquefaction process which combines autorefrigeration with one or more closed-loop multicomponent refrigeration systems.